1. Field of the Invention
The present invention relates to an active material for a positive electrode (positive electrode-active material) and a lithium cell comprising the same. In particular, the present invention relates to a positive electrode-active material with a high voltage and a high capacity and a lithium cell comprising the same.
2. Prior Art
Hitherto, a lithium cell comprising copper oxide (CuO) as a positive electrode-active material (hereinafter referred to as a xe2x80x9cLi/CuOxe2x80x9d cell) has been proposed as a lithium cell having a high capacity density (see Brounssely M., Jumel Y. and Cabano J. P., 152nd Electrochemical Society Meeting, Atlanta (1977)).
However, the Li/CuO cell has a low closed circuit voltage of about 1.2 to 1.5 V, and thus it cannot be used in applications which require a high voltage.
One object of the present invention is to provide a positive electrode-active material which can achieve a high voltage without decreasing the capacity, and a lithium cell comprising such a positive electrode-active material.
Accordingly, the present invention provides a positive electrode-active material consisting of at least one compound selected from the group consisting of a copper-boron double oxide of the formula:
CujMkBmOnxe2x80x83xe2x80x83(I)
wherein M is a metal atom, and j, k, m and n are each a positive integer, a copper-molybdenum double oxide of the formula:
CuxMoyOzxe2x80x83xe2x80x83(II)
wherein x, y and z are each a positive integer, and a copper double oxide of the formula:
CuMxe2x80x22O4xe2x80x83xe2x80x83(III)
wherein Mxe2x80x2 is at least one element selected from the group consisting of B, Al, Ga, Mn, Co, Ni and a rare earth element Ln (for example, Y, La, etc.)
Furthermore, the present invention provides a lithium cell comprising a positive electrode comprising the above active material of the present invention, a lithium negative electrode, and an electrolyte solution.
In the lithium cell of the present invention, for example, a Li/Cu2FeBO5 cell, unlike the conventional Li/CuO cell, the circumstance around the reducing copper atom of Cu2FeBO5 is different from that of CuO, and thus the distance of the Cu-O bond varies. In addition, the Cu atom is bonded with Fe and B. Therefore, the lattice energy is increased. It is known that the increase of a lattice energy and the variation of the distance between a reducing atom and the nearest atom increase a discharge voltage (Masayuki Yoshio and Akiya Ozawa Ed., xe2x80x9cLithium Ion Secondary Cellsxe2x80x9d, page 8 (published by Nikkan Kogyo Shinbunsha), 1996).
In the synthesis of the positive electrode-active material such as Cu2FeBO5, CuB2O4, Cu3Mo2O9, etc., since B2O3, H3BO3 or MoO3 having a low melting point is used, Cu2FeBO5, CuB2O4 or Cu3Mo2O9 of a single phase can be synthesized at a relatively low temperature, and the particles of Cu2FeBO5, CuB2O4 or Cu3Mo2O9 having a small particle size can be produced by the synthesis at such a low temperature. The use of such Cu2FeBO5, CuB2O4 or Cu3Mo2O9 having a small particle size as a positive electrode-active material can increase the conductivity of the positive electrode and, in turn, the discharge voltage. In the course of discharging, the Cu2FeBO5 or CuB2O4 particles may generate B2O3 or boron compounds and Fe2O3, and Cu3Mo2O9 may generate MoO3, and such generated materials discharge in the range between 1 V and 3 V. Thus, the capacity of the cell further increases. Therefore, in cooperation with the above-described increase of the discharge voltage, a lithium cell having a high voltage and a high capacity can be obtained.
FIGS. 3 and 4 show the discharge characteristics of the cell of Reference Example 1 comprising B2O3 as a positive electrode-active material, and that of the cell of Reference Example 2 comprising MoO3 as a positive electrode-active material, respectively. From FIGS. 3 and 4, it can be understood that the cells of Reference Examples 1 and 2 discharge at a higher voltage than a cell comprising CuO as a positive electrode-active material, the discharge characteristics of which are shown in FIG. 2.